In recent years, biodegradable plastics have been actively developed as materials that can solve problems caused by the heavy burden of plastic waste on the global environment, such as harmful effects on the ecosystem, generation of harmful gas during combustion, and global warming due to a large amount of heat generated by combustion.
Particularly, carbon dioxide generated by combustion of plant-derived biodegradable plastics was originally present in the air. Therefore, combustion of plant-derived biodegradable plastics does not increase the amount of carbon dioxide in the atmosphere. This is referred to as “carbon neutral”, and is regarded as important under The Kyoto Protocol that sets targets for reducing carbon dioxide emissions. Therefore, active use of plant-derived biodegradable plastics is desired.
Recently, from the viewpoint of biodegradability and carbon neutral, aliphatic polyester-based resins have received attention as plant-derived plastics. Particularly, polyhydroxyalkanoate (hereinafter, sometimes referred to as PHA)-based resins have received attention. Among PHA-based resins, poly(3-hydroxybutyrate) homopolymer resins (hereinafter, sometimes referred to as P3HB), poly(3-hydroxybutyrate-co-3-hydroxyvalerate) copolymer resins (hereinafter, sometimes referred to as P3HB3HV), poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) copolymer resins (hereinafter, sometimes referred to as P3HB3HH), poly(3-hydroxybutyrate-co-4-hydroxybutyrate) copolymer resins, polylactic acid, etc. have received attention.
However, it is known that such PHA-based resins are hard resins and become brittle with time after mold processing due to their very slow crystallization.
Generally, a plasticizer is added to impart flexibility to a hard resin. This, however, involves a problem that bleeding occurs due to the use of a large amount of plasticizer.
Patent Document 1 discloses, as the technique of imparting good impact strength, blending of a poly(hydroxyalkanoic acid) with an ethylene-vinyl acetate copolymer. This document states that the vinyl acetate content of the ethylene-vinyl acetate copolymer is 6 wt % or more. However, compatibility and improving effect vary depending on the combination of the vinyl acetate content of the ethylene-vinyl acetate copolymer and the type of poly(hydroxyalkanoic acid) used, and therefore impact strength is insufficiently improved.
Patent Document 2 discloses a composition obtained by blending biodegradable P3HB3HV, an ethylene-vinyl acetate copolymer having a vinyl acetate content of 5 to 30 wt %, etc. and states that a certain percentage of the composition has biodegradability, and the elastic modulus or breaking elongation of the composition can be controlled to be in a certain range. However, the copolymer and P3HB3HV are incompatible. Further, in order to impart sufficient ductility to the composition containing P3HB3HV, the ethylene copolymer needs to be blended so as to account for almost half of the total amount of the composition, which reduces biodegradability.
Patent Document 3 discloses a composition containing a lactic acid-based polymer and an ethylene-vinyl acetate copolymer having a vinyl acetate content of 30 to 90 wt %, and states that blocking between the layers of a resulting film can be prevented, and the composition has good breaking elongation. However, satisfactory breaking elongation is not necessarily obtained.
Patent Document 4 discloses a heat-shrinkable laminate film comprising: a layer comprising a resin composition containing a polylactic acid-based resin as a main component and an ethylene-vinyl acetate copolymer-based resin; and a layer comprising a resin composition containing a polylactic acid-based resin, a polyolefin-based resin, and a compatibilizer that improves compatibility between the polylactic acid-based resin and the polyolefin-based resin. This document states that a heat-shrinkable laminate film having excellent ductility can be obtained by laminating at least the above-described two different layers. However, the layer comprising a resin composition containing a polylactic acid-based resin as a main component and an ethylene-vinyl acetate copolymer-based resin cannot necessarily have satisfactory ductility as a film by itself.
On the other hand, the above-described PHA-based resins are slow in crystallization, and therefore require a long cooling time for solidification after heat-melting in mold processing, which causes problems such as poor productivity and temporal change in mechanical properties (especially, toughness such as tensile elongation at break) due to secondary crystallization that occurs after molding.
Therefore, blending of a PHA-based resin with an inorganic material such as boron nitride, titanium oxide, talc, lamellar silicate, calcium carbonate, sodium chloride, or metal phosphate has heretofore been proposed to promote crystallization. However, the blending with an inorganic material has many adverse effects on a resulting molded article, such as reduction in tensile elongation and poor appearance, and is therefore poorly effective.